Diodes are well-known in the art of electronic devices. Resonant tunneling diodes are characterized by non-linear current-voltage (I-V) relationships which display a negative differential resistance over a known domain of voltage values. Negative differential resistance exists when, over a limited range of current values, the current exhibits a decrease in value while, over the corresponding voltage domain, the value of the voltage is increased. Diodes known in the art include semiconductor tunnel diodes and voltage controlled RTDs which have been employed as switches and oscillators, among other applications.
A voltage controlled resonant tunneling diode is a variety of RTD characterized by a negative differential current-voltage relationship when the voltage is swept through a wide range of values and the current is measured. Specifically, over a known current range, RTDs show an actual decrease in the value of the current when the voltage values are increased indicating a negative differential resistance over this domain of voltage values.
Typically, prior art RTDs have comprised a double barrier-quantum well structure. In one example, a gallium arsenide quantum well layer will have relatively thin barriers of aluminum arsenide epitaxially joined to each side of the quantum well layer. The resulting structure will then be placed between two injection layers comprised of gallium arsenide. These injection layers provide a reservoir of electrons for the device.
In prior art RTDs, the barrier layers prevent the free flow of electrons through the device when a voltage is applied to the diode. When a voltage is applied to the diode, electrons are injected into the quantum barrier from the conduction band of the injection layer on the negatively biased side of the device. Only those injected electrons having specified energies can tunnel through the barrier layers and the quantum well layer. The requirement that electrons meet the tunneling conditions gives rise to the negative differential resistance characteristic over a range of voltages.
It has been proposed that incorporation of RTDs into such circuits as high speed signal processors, high frequency oscillators, flip flops and the like will result in speeds of operation of these devices currently unreachable by conventional semiconductor devices, e.g., bipolar transistors. An essential feature of key electronic components, such as flip-flops and switches is current bistability in the I-V relationship, which provides two stable state for binary logic. Current bistability exists when more than one stable value of current exists for a particular voltage value.
Previously known RTDs tend to exhibit voltage bistability, having more than one voltage value for a particular value of the current. Circuits such as those mentioned above which have incorporated RTDs have typically included a large load resistance in combination with the RTD to create the desired current bistability. Thus, the load resistances are used to convert a voltage bistability into a current bistability. However, use of these load resistances imposes a penalty on device speed.
Therefore, it is desirable to have devices which eliminate the need to include such resistances when incorporating RTDs into electronic components. An RTD which makes the elimination of large load resistances possible must exhibit the current bistability that leads to the desired enhancement of speed of operation.